Resistive fluid detecting means in the form of probes to sense the presence or absence of a resistive fluid in a container have been known for many years. One widely used type of installation that relies on resistive fluid detecting means are boilers. The presence or absence of boiler water in a heating plant boiler can be monitored by a resistive fluid detecting probe. This type of probe normally relies on the establishment of a single conductive circuit between the probe and the boiler itself. A resistance measurement is then converted into a decision as to whether or not the boiler water is present in the boiler. In some systems, two probes are used to establish two different levels of water in a boiler, thereby establishing a differential between the need to add water to the boiler and the normal content of water within the boiler. The establishment of the differential is desirable in order to eliminate the need for cycling a pump or solenoid valve when a ripple or slight movement of the boiler water level occurs at the end of a probe.
Boiler water sensing mechanisms also have been developed which utilize floats to sense the level of boiler water and mechanically operate switches. This type of mechanism is subject to wear and boiler water scale contamination, thereby creating a sensing mechanism which may be more complex and less desirable than a simple resistive fluid detecting type of probe.